Fuel tanks used to provide fuel for internal combustion engines may under some conditions have a positive internal pressure. This can occur, for example, when the fuel tank is filled with liquid fuel at a relatively lower temperature and the temperature of the tank and its contents then increases such as by exposure of the fuel tank to a warmer ambient temperature and/or to sunlight. Since the liquid fuel and the fuel vapors in the tank will seek to expand as their temperature increases, this expansion may increase the positive internal pressure in the fuel tank.
If the fuel tank is connected by a fuel supply line to an internal combustion engine, it is desirable to prevent fuel and/or fuel vapors in the tank under this positive pressure condition from flowing to the engine and potentially contaminating the environment when the engine is not operating. Prior art valves for this purpose are provided in the fuel supply line. These valves are spring biased to a normally closed position and prevent fuel and/or fuel vapor under positive pressure from flowing from the fuel tank through the valve when the engine is not operating.
When the engine is to be started, the normally closed valve is opened in response to a demand for fuel to allow fuel to be supplied to the engine from the fuel tank. The valve is referred to as a demand valve and is opened by a negative pressure (or vacuum) condition that is applied to the downstream side of the valve during starting and during operation of the engine. Fuel then flows through the opened valve to the engine. To apply the negative pressure condition to the downstream side of the demand valve before starting the engine, a primer bulb or other pumping device is provided in the fuel supply line between the demand valve and the engine. Operation of the priming bulb creates a negative pressure on the downstream side of the priming bulb to open the demand valve and allow fuel flow to the engine. As the engine is turned to start it and after it is started, the engine continues to create the negative pressure condition on the downstream side of the demand valve to keep the demand valve opened. When the engine is stopped, the negative pressure on the demand valve is abated or no longer applied and the demand valve again closes to prevent fuel flow to the engine until the demand valve is reopened by the primer bulb. Valves of this general type that minimize fuel and/or fuel vapor leakage from fuel tanks are currently required in the United States for certain marine applications, to minimize escape of fuel and/or fuel vapors from the tank when its associated engine is not running.